knocking sound at idle

[montanakeno]

Hi all, can someone help...i have a 4.0 and it has a knocking sound at idle....would replacing the main bearings in the bottom help?

[dnparadice]

I have had a 91 that did that I got the engine rebulilt at 195,000 miles and it solved that and many other problems. I have a 93 with 160,000 that knocks but runs great otherwise, It has made the same sound since 110,000. I got used to the noise maby I will rebuild it too if any serious problems arise.
-JC

[MagicRat]

Originally Posted by montanakeno

Hi all, can someone help...i have a 4.0 and it has a knocking sound at idle....would replacing the main bearings in the bottom help?

These engines are notorious for knocking yet working well, for a long time.
I might suggest hooking up a mechanical oil pressure gauge and see what the pressure is like. If its much below 20 lbs at idle, change the pump. If its good, then an engine rebuild is in order, New bearings are limited in use without measuring the crank and regrinding it if required. And if you go that far, just rebuild the whole engine.

[JeEpBoY95]

i had a 93 with a 4.0L with 110,000 that knocked but when we tore down to rebuild there was minimal wear on the bearings, pistons, crank and head......it turned out to be a cracked exsaust manifold that was making the noise

[b1gfella]

same thing here...94 with 125k and it has knocked since i got it at 25k...it is a known issue with these engines and they tried to fix it several times in the engine redesigns but it just keeps happening...nothing to worry about if it is the "normal" knocking sound we all know and love

[mybicepsarehuge]

ya man i have an 88' xj with the 4.0...it sounds like a minimal low sounding knocking ...might be the exhaust...has over 317,000 kms on her though! i want to rebuild it top end to bottom..anyone have a good guess on how much it costs??

*cheers*

[markman]

Originally Posted by montanakeno

Hi all, can someone help...i have a 4.0 and it has a knocking sound at idle....would replacing the main bearings in the bottom help?

Your problem is most likely due to excessive carbon build up on your pistons. I have a 1990 Cherokee that had the same problem. A friend turned me on to a quick fix; you get the engine good and warm, then spray carborator cleaner into the intake until the engine dies. Then you wait about 30 minutes then restart the engine. Then take a spray bottle of water and spray about 3 or 4 good sprays of water into the intake, then gun the motor real good a few times. The water turns into steam in the cylinders and since the carborator cleaner already loosened the carbon on the pistons, the steam finishes the process. This may sound weird, but I promise you it will work!!